1872.] between Explosions in Collieries and Weather. 301 



when it starts from the end of the in-take air-course on its passage 

 through the workings, and that for every equal space it travels between 

 the in-take and return air-course it receives an equal quantity of gas. 

 Thus when it arrives at the return air-course it consists of a mixture 

 of air and fire-damp, whose constitution depends on the quantity of air 

 passing through the workings, on the rate at which gas escapes into it, 

 and on the distance between the in-take and return air-course. 



It is therefore evident that if either the supply of air be diminished or 

 the supply of gas increased, the resulting mixture will be rendered more 

 explosive, not only in the return air-course, but also at every point of the 

 passage between the in-take and return air-course. 



If, then, from any causes the mixture shall have reached the firing-point 

 when it enters the return air-course *, any aggravation of these causes 

 would make the firing-point to travel backwards through the workings 

 towards the in-take air-course. In this manner the ventilating-current 

 may itself become explosive in some parts of its course. Again, although 

 the ventilating-current itself may never become explosive, its gradual fouling 

 may cause explosive mixtures to be generated in certain places, such as 

 unbratticed bords, recesses between pack-walls, and cavities in the roof in 

 the following manner (all other cases may be, more or less directly, re- 

 ferred to these) : — 



Fie. I. 



Figs. 1 and 2 represent sections of the workings. 11 is the roof, F the 

 floor, C is a point where the ventilating-current passes in a direction 

 normal to the plane of the paper. In fig. 1, a b c is a cavity in the roof 



* Although the air entering any particular return air-course may be explosive, it will 

 probably become diluted again before it reaches the upcast, by leakages from the in-take 

 air-course and admixture of air from other districts. 



